Like you said, it could be just the old seal on the gas cap, but it could also be clogged PCV system hoses or the nipple on this system which is at the base of the large hose that goes from the MAF to the turbo (several visible connectors at the base of this hose). Check under the car and see if you have blow-by at the turbo's side...an indication of this issue. Make sure it's not the turbo's oil return line leaking...also a common issue, but would not cause the vacuum line failure notice. Also, another possibility is the oil trap being clogged. Either of these are messy and a real PIA to deal with. If you have a good indy mechanic, get him to check these for you. I'm currently in the middle of changing out my PCV system hoses as well as my oil trap on my S70...not setting any codes, but I have the blow-by at the side of my turbo, so decided I should do this...It's several hours of work as the intake manifold has to come loose to get to the oil trap under it. Good luck.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.